Generally speaking, loudspeakers are sound-generating instruments that convert an electrical signal into sound. Loudspeakers which are applied in small-sized portable electronic devices such as mobile phones are sometimes referred to as micro speakers as their size is small. As small-sized portable electronic devices, e.g. mobile phones, are becoming more and more miniaturized, these micro speakers are also more and more miniaturized to adequately correspond to this ongoing trend of miniaturization.
FIG. 1 illustrates a state of the art micro speaker, which may be used in some existing portable electronic devices. This micro speaker is composed of a housing 1, a magnet 2 contained in the housing, a voice coil 3 and a membrane 4. The speaker generates sound as voice coil 3 moves vertically up and down owing to a magnetic field formed by the interaction between voice coil 3 and magnet 2 if electrical current is applied at voice coil 3 through lead wire from outside. As voice coil 3 moves up and down according to magnetic field variation applied at voice coil 3, the membrane 4 attached to the voice coil 3 vibrates up and down to generate sound. The micro speaker shown in FIG. 1 is only capable of providing a loudspeaker function.
Vibration means have recently been furnished at the same time together with the above-mentioned micro speaker in a portable electronic device to provide users with a vibration function in addition to the sound-generating function provided by the speaker. This way, users may be alerted not only by sound but also by vibration. The vibration means often used in the known prior art is a vibration motor. However, because there is a continuing need for miniaturization of portable electronic devices, the relatively bulky vibration motors may be undesirable in some applications. An alternative vibration speaker solution has been disclosed in U.S. Pat. No. 6,487,300 B1, which describes a vibration speaker contained in a portable communication instrument such as a mobile phone.